Document the prdb (ubik) file format

Briefly cover the ubik header and mention that it is not part of the
logical database (since it is just used for the consistency procedure).

Describe the fields of the prheader and how they are used.  Mention that
all subsequent entries are blocks of the same size, whose type can be
distinguished by a shared flags field.  User and group entries are similar,
and supergroup entries are described as a diff from regular group entries,
as only a handful of fields change.  Continuation entries can be used
for user, regular group, or supergroup entries.

Call out what fields are invariant within which classes of entry, so that
these properties can be preserved (or knowingly eliminated) for future
extensions to the format.

Change-Id: Id5ca7d8d346c09c2eec50691e5c5fefbe735ac60
Reviewed-on: http://gerrit.openafs.org/10022
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>